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“Their endorsement is not just a paper endorsement,” said Sanders, at a joint Washington press conference with CWA – the third union to support his bid. “We’re going to have thousands of people on the ground knocking doors, making phone calls and helping us.”

Sanders has sought to project new energy behind his campaign, as national polls show him trailing Democratic front-runner Hillary Clinton by double-digits. His team announced Thursday it had received 2 million financial contributions from supporters – underscoring Sanders’ online fundraising successes. Sanders’ campaign said it has raised more than $3 million since Monday.

The union endorsement was widely expected.

The organization’s former president, Larry Cohen, has been an unpaid labor adviser to Sanders’ campaign and the CWA has been a vocal opponent of the Trans Pacific Partnership trade agreement, which Sanders has assailed as detrimental to middle-class jobs and wages.

“We will use whatever we need” to drive the campaign, said President Chris Shelton. “We will use our PAC money. If Bernie doesn’t want to take it, ok, I respect that, but we'll use it to make sure we'll do everything we can to get the vote out.”

The group decided to back Sanders based solely on a vote by its members, rather than a decision by their leader.

Unlike many other unions, CWA did not ask Sanders to answer a questionnaire or sit for interviews with their executive board. Their leadership instead helped develop questions for a live interview conducted at the AFL-CIO summer meeting.

“It would have been an empty endorsement coming from me,” said Shelton. “He could have gotten 22 votes from our executive board. This way, he will have 700,000 votes.”

Shelton would not release the exact percentage of members who backed his campaign in the vote.

While many union leaders have declared their support for Clinton, in some cases, rank-and-file members have been more drawn to Sanders.

“What I would have hoped is that unions who believe in democracy would have done what the CWA did, which is really create a wide-open process,” said Sanders. “We would have a lot more national union support than is currently the case.”

In 2012, tens of thousands of CWA volunteers in 38 states worked on national and state campaigns, including in presidential battleground states like Florida, Colorado, Virginia, and Ohio.

Clinton has locked up nearly 20 labor endorsements accounting for about 11 million members, including influential unions, such as the National Education Association and the Service Employees International Union. Last month, she announced “Hard Hats for Hillary,” a coalition organizing support for Clinton from millions of construction, building, transportation and other labor industry professionals.

“She shares their commitment to fighting for an economy that works for every single American, not just those at the top,” said Clinton campaign spokesman Jesse Ferguson.

Sanders was also endorsed by Democracy for America, a liberal grassroots organization that grew out of former Vermont Gov. Howard Dean’s 2004 presidential bid.

The group attempted to draft Massachusetts Sen. Elizabeth Warren into the presidential campaign earlier this year and many of its backers migrated to Sanders’ campaign after Warren opted not to run. DFA said nearly 90 percent of its membership backed Sanders – its first presidential primary endorsement.

The organization is chaired by Jim Dean, the ex-governor’s brother. But Howard Dean has endorsed Clinton and has been an active surrogate for her campaign in the early voting states.

DFA, which cites a membership of 1 million, said it would run a “100 percent positive campaign” in support of Sanders and would back any candidate who secures the Democratic nomination.

Clinton, meanwhile, picked up the support of the Chair of the Congressional Hispanic Caucus, Rep. Linda Sanchez, of California, in a Spanish-language op-ed.

“Lasting change was bound to take more than a single presidency,” she wrote. “It will take a multi-generational effort, led by someone with the skills and the scars that making real progress often demands.”